Many fully integrated receivers of digital and/or analog radio broadcast signals use low noise amplifiers (LNA) in the input stage for enhancing sensitivity of the receiver. Often the LNA represents the interface between the receiver, integrated on silicon, and the external circuitry (antenna, filters, etc.) operating at radio frequency (RF). Normally, in RF applications the common functional blocks have an input and/or output impedance of 50 .OMEGA..
In consideration that in silicon integrated systems the signal path lengths are negligible compared to the wavelength, only the input stage must have an input impedance of 50 .OMEGA. because the output of the LNA is directly fed to the co-integrated mixer. In integrated high frequency amplifiers, high gain and low noise characteristics are difficult to achieve with acceptable return losses. This imposes the need for an external impedance matching network which increases the complexity and cost of the whole receiver.
A rather common application is that of a superetherodyne receiver and the ensuing description will at times refer to this important application. A typical superetherodyne receive chain is shown by way of the block diagram of FIG. 1. This type of receiver is currently used in most mobile radio apparatuses.
The RF signal sensed by the antenna is mixed with a certain frequency, generated by a local oscillator (LO). Commonly, the signal coming from the antenna passes through a preselector filter to reduce the out-of-band noise. The low noise amplifier (LNA) increases the power level of the RF signal to an amplitude suitable to convert the signal to an intermediate frequency. The mixer stage mixes the RF antenna signal with the signal generated by the local oscillator LO, outputting an intermediate frequency signal (IF).
In a well-designed receiver, the noise performance is largely dependent on the characteristics of the first amplifying stage, as well as on the losses that occur upstream of the LNA. This means that the LNA must have a high gain and low noise properties. In addition, any filter and/or network present upstream of the LNA input must cause the lowest possible insertion losses.
The noise introduced by the LNA amplifier depends primarily on the geometry of the active devices, on the quiescent current and on the input impedance. Therefore, when designing low noise integrated AC amplifiers, great attention must be devoted to the input stage.
The noise figure (NF) requirement, normally fixed by the system specifications, dictates the area of the input transistors and their collector current (Ic). Low noise amplifiers usually show a poor return loss from their input stage. To improve this intrinsic characteristic of an integrated LNA, use is made of external impedance matching circuits which, in turn, may cause insertion losses, according to the scheme shown in FIG. 1.
Another typical drawback of fully integrated receivers comes from the fact that analog circuit blocks (amplifiers) that are relatively close to the local oscillator, commonly a voltage controlled oscillator (VCO), and thereby to digital circuit blocks functioning in the baseband frequency, are subject to an injection of switching noise. This comes from logic circuits switching at relatively high levels through the common substrate and supply rails. This type of noise seriously compromises the noise performance of the analog blocks of the receiver. To overcome this situation a differential configuration is mandatory for those blocks that are most seriously affected by switching noise.
Moreover, prevailing market requirements are for fully integrated receivers provided by circuit blocks capable of functioning at particularly low supply voltages (&lt;2.5V) for battery powered portable devices. The relatively low supply voltage restricts the dynamic range and renders the biasing conditions of an LNA more critical. As a result, it becomes even more difficult to satisfy both the high gain and low noise specifications using traditional circuit configurations for these types of low noise amplifiers.
There are many known circuits for implementing a low noise amplifier. In fully integrated low noise amplifiers for RF applications it is common to use resistive loads rather than inductive loads. Inductive loads are strongly dependent upon the frequency of operation and accurate Q figures to achieve a high gain. A resistive load allows for a more accurate control of the gain.
The basic circuit of a typical fully differential LNA is depicted in FIG. 2, wherein the equivalent intrinsic noise voltage and the current source value are also indicated. The circuit has a cascoded structure to ensure an enhanced frequency response. It is easy to notice that the main parameters of the amplifier of FIG. 2, such as the noise Figure, the input impedance and the gain are respectively given by: ##EQU1##
Rs is the source resistance (typically 50 .OMEGA.), g.sub.m is the transconductance of the transistors of the input pair, Q1 and Q2, r.sub.b is the base resistance of the same transistors Q1 and Q2 (typically of about 8 .OMEGA.), .tau..sub.F is the transit time of Q1 and Q2 (typically around 6 picoseconds), and Av is the voltage gain (typically about 20 dB).
For example, by solving equation (1) for a typical value of Ic=2 mA, we obtain:
NF=1.2 dB PA1 Rc=130 .OMEGA. PA1 Zin=650 e.sup.-j70 .OMEGA.
These Figures show that the noise figure has an excellent value although it is attained at the expense of a relatively high input mismatch. Indeed, by directly driving the amplifier, without using an impedance matching network, the return loss, L, at 1 GHz may be easily computed to be about 0.5 dB, which is a definitively poor value.
To increase the return loss figure, an external matching network must be used. The external matching network, in turn, increases the noise figure as well as the complexity and the cost of the system.
Moreover, the above discussed known configuration is also affected by the DC drop on the load resistances Rc. This limits the ability of the amplifier to function at a reduced supply voltage. Indeed, the direct current crossing the input pair of transistors Q1 and Q2, which must be relatively high to ensure a sufficient low noise value, also flows through Rc, causing a consequently large voltage drop.